Moire fringe measurement systems are commonly used when three-dimensional measurements are to be made on almost flat objects. True interferometry, is used when the deviation from flat is comparable in size to the wavelength of light, but this technique is normally much too fine for engineering purposes The Moire fringe technique allows much coarser measurements to be made. The technique operates by projecting a pattern of fine equally spaced lines onto the object under test and viewing the reflected image with a camera which has a grid over its sensor, such that for a perfectly flat object the image of the bright lines always falls on the sensor grid and not en the spaces between the grids, so that the image light does not access the sensitive surface of the sensor. Any deviation from flat causes some of the image of the bright lines to fall on the sensor surface. The deviations are thus recorded as lighter areas. For larger deviations, the image of the bright line will again fall on an opaque grid. The net result of this is that a contour plot type of result can be obtained with each contour corresponding to one period of the grid. How this relates to actual depth is determined by the spacing of the grids, the focal lengths of the imaging systems and the angle of incidence of the projection and viewing system. The grid need not be a set of lines but may be any pattern by which an image can be transmitted onto the surface of the object. By careful selection, the pattern can be used for special purposes such as varying the resolution of the field of view. By the use of different grids, it is possible to produce a wide range of sensitivities more appropriate to engineering uses than purely optical interference systems, and also not needing monochromatic or near monochromatic light as do purely optical interference systems.
A schematic of a typical design of a prior art Moire system, is shown in FIG. 1 of the accompanying drawings. A grid 1 is imaged on a test piece 2 by means of projection optics 3. A second grid 4 is used in the imaging system 5, and this grid is re-imaged on the photo-sensor of a camera 6. It is usually not practical to place the grid 4 directly on the sensor. To use the system, the projection and imaging optics must first be carefully aligned by use of a flat test object. There are very many degrees of freedom in the system and, in order for it to work efficiently, the projections of the two grids on the test object must coincide exactly. Taking the camera as fixed, the projection optics 3 must be correctly aligned in three angles and two distances. All the angles and distances are not along convenient orthogonal axes. If adjustments using axes orthogonal to the test piece are used, the adjustments will be highly interactive and hence difficult and time consuming. A further significant disadvantage of the system is that it needs two grids. To change the grid spacing thus requires two new grids, and again the grids must be accurately aligned with one another. Generally, the use and especially the setting up of such a system is much more complicated and difficult than most industrial users are prepared to tolerate.